A node-based informed modularity strategy to identify organizational modules in anatomical networks

Anatomical network modules of the human central nervous-craniofacial skeleton system

Anatomical network analysis (AnNA) is a systems biological framework based on network theory that enables anatomical structural analysis by incorporating modularity to model structural complexity. The human brain and facial structures exhibit close structural and functional relationships, suggestive of a co-evolved anatomical network. The present study aimed to analyze the human head as a modular entity that comprises the central nervous system, including the brain, spinal cord, and craniofacial skeleton. An AnNA model was built using 39 anatomical nodes from the brain, spinal cord, and craniofacial skeleton. The linkages were identified using peripheral nerve supply and direct contact between structures. The Spinglass algorithm in the igraph software was applied to construct a network and identify the modules of the central nervous system-craniofacial skeleton anatomical network. Two modules were identified. These comprised an anterior module, which included the forebrain, anterior cranial base, and upper-middle face, and a posterior module, which included the midbrain, hindbrain, mandible, and posterior cranium. These findings may reflect the genetic and signaling networks that drive the mosaic central nervous system and craniofacial development and offer important systems biology perspectives for developmental disorders of craniofacial structures.

Redefining the simplicity of the craniomandibular complex of nightjars: The case of Systellura longirostris (Aves: Caprimulgidae) by means of anatomical network analysis

To study morphological evolution, it is necessary to combine information from multiple intersecting research fields. Here, we report on the structure of the bony and muscular elements of the craniomandibular complex of birds, highlighting its morphological architecture and complexity (or simplification) in the context of anatomical networks of the Band-winged Nightjar Systellura longirostris (Caprimulgiformes, Caprimulgidae). This species has skull osteology and jaw myology that departs from the general structural plan of the craniomandibular complex of Neornithes and is considered morphologically simple. Our goal is to test if its simplification is also reflected in its anatomical network, particularly in those parameters that measure complexity and to explore if the distribution of the networks in a phylomorphospace is conditioned by their evolutionary history or by convergence. Our results show that S. longirostris clusters with other Strisores and momotids and is segregated from the other bird species analyzed when plotted in the phylomorphospace, as a consequence of convergence in the network parameters. Systellura has a craniomandibular complex consisting of fewer muscles connecting more bones than the model species (e.g., the rock pigeon or the guira cuckoo). In this sense, Systellura is actually more complex regarding the number of integrative bony parts, while its craniomandibular complex is simpler. According to its anatomical network, Systellura also can be interpreted as less complex, particularly compared with other Strisores and taxa that reflect the general structure of the craniomandibular complex in Neornithes.

Breaking the mold: telescoping drives the evolution of more integrated and heterogeneous skulls in cetaceans

Background

Along with the transition to the aquatic environment, cetaceans experienced profound changes in their skeletal anatomy, especially in the skull, including the posterodorsal migration of the external bony nares, the reorganization of skull bones (= telescoping) and the development of an extreme cranial asymmetry (in odontocetes). Telescoping represents an important anatomical shift in the topological organization of cranial bones and their sutural contacts; however, the impact of these changes in the connectivity pattern and integration of the skull has never been addressed.

Methods

Here, we apply the novel framework provided by the Anatomical Network Analysis to quantify the organization and integration of cetacean skulls, and the impact of the telescoping process in the connectivity pattern of the skull. We built anatomical networks for 21 cetacean skulls (three stem cetaceans, three extinct and 10 extant mysticetes, and three extinct and two extant odontocetes) and estimated network parameters related to their anatomical integration, complexity, heterogeneity, and modularity. This dataset was analyzed in the context of a broader tetrapod skull sample as well (43 species of 13 taxonomic groups).

Results

The skulls of crown cetaceans (Neoceti) occupy a new tetrapod skull morphospace, with better integrated, more heterogeneous and simpler skulls in comparison to other tetrapods. Telescoping adds connections and improves the integration of those bones involved in the telescoping process (e.g., maxilla, supraoccipital) as well as other ones (e.g., vomer) not directly affected by telescoping. Other underlying evolutionary processes (such as basicranial specializations linked with hearing/breathing adaptations) could also be responsible for the changes in the connectivity and integration of palatal bones. We also find prograde telescoped skulls of mysticetes distinct from odontocetes by an increased heterogeneity and modularity, whereas retrograde telescoped skulls of odontocetes are characterized by higher complexity. In mysticetes, as expected, the supraoccipital gains importance and centrality in comparison to odontocetes, increasing the heterogeneity of the skull network. In odontocetes, an increase in the number of connections and complexity is probably linked with the dominant movement of paired bones, such as the maxilla, in retrograde telescoping. Crown mysticetes (Eubalaena, Caperea, Piscobalaena, and Balaenoptera)are distinguished by having more integrated skulls in comparison to stem mysticetes (Aetiocetus and Yamatocetus), whereas crown odontocetes (Waipatia, Notocetus, Physeter, and Tursiops) have more complex skulls than stem forms (Albertocetus). Telescoping along with feeding, hearing and echolocation specializations could have driven the evolution of the different connectivity patterns of living lineages.

Reduction, reorganization and stasis in the evolution of turtle shell elements

Novel phenotypic configurations can profoundly alter the evolutionary trajectories of species. Although innovation can precede lengthy periods of evolutionary stasis, the potential for species to diversify further can be realized via modular changes across distinct levels of hierarchical organization. To test this expectation, we undertook anatomical network analyses to model the organization and composition of the turtle’s shell. Our results suggest that stem turtles featured the greatest diversity in the number of skeletal (bones) and epidermal (scutes) shell elements. The shell subsequently underwent numerical simplification. Thus, the sum of potential connections (links) in shell networks has diminished in modern turtles. Some network system descriptors of complexity, integration and modularity covaried with the number of network components (nodes), which has remained evolutionarily stable since the Jurassic. We also demonstrated that shell reorganization might be feasible within modular subdivisions, particularly in modern turtles with simplified and less integrated network structures. We discuss how these findings align with previous studies on numerical simplification with enhanced skeletal specialization in the tetrapod skull. Altogether, our analyses expose the evolvability of the turtle’s shell and bolster the foundation for further macroevolutionary comparisons of ancient and modern species.